The present invention refers to a lead dioxide-zinc rechargeable-type cell in which the positive and negative plates remain submerged in the electrolyte and, more particularly, it is related to a lead dioxide-zinc rechargeable-type cell which plates are permanently submerged in an electrolyte which provides sufficient aggressivity against the anode to attack the same at closed circuit but not sufficiently aggressive to attack the same at open circuit.
Heretofore it has been well known to build rechargeable-type cells comprising a positive plate of lead dioxide and a negative plate of lead and designed so that the electrolyte can be introduced before placing the cell in service. This type of rechargeable cells can be stored in the unactivated state indefinitely without requiring special precautions against deterioration and have an exceptionally good watt-hour capacity per unit of cell weight and operate satisfactorily at low temperatures.
The very well known lead storage battery described above, however, has shown very serious drawbacks, such as the fact that the voltage obtained thereby is relatively low and does not exceed two volts while, on the other hand, the utilization of the lead dioxide is limited by the formation of insoluble lead sulfate which gradually accumulates in the cell rendering it useless in a relatively short time when care is not exercised such that the drainage of the cell is not sufficiently great to cause excessive formation of insoluble lead salts.
Regardless of the above disadvantages shown by the lead storage battery which has been of common use heretofore, use of these type of batteries has not been abandoned in view of the fact that there is no efficient substitute therefor, inasmuch as the prior art lead dioxide-zinc type cells have been considered to be useful only for discharges of short duration, despite the fact that lead dioxide-zinc cells provide a relatively high electromotive force such as, for instance, 2.5 volts and higher. In other words, prior art lead dioxide-zinc cells have been considered to be useless in prolonged discharges, inasmuch as the electrolyte provided in these cells (commonly sulfuric acid) in which lead salts are freely soluble is obviously precluded for cells designed for such prolonged discharges, due to the aggressivity of said electrolyte against zinc metal.
Therefore, for long it has been thought that for services requiring prolonged discharges, lead was the most desirable metal for the negative plates, inasmuch as cells with negative plates containing active metals more noble than lead have, of course, a low electromotive force. As zinc is an active metal less noble than lead, it has been thought that this couple, which provides a much higher electromotive force, should be considered as highly suitable. However, zinc does not provide for a rechargeable characteristic of the cells in the presence of a sulfuric acid electrolyte in view of the violent evolution of hydrogen which prevents redepositing of the zinc and, on the other hand, the cells cannot be stored with the plates submerged in the electrolyte, in view of the fact that the common sulfuric acid type electrolytes are too aggressive for the zinc metal, whereby they attack the negative plate easily even at open circuits.
Heretofore the only cells which have been put in service for limited applications are irreversible type cells comprising the zinc-sulfuric acid-lead dioxide system (W. J. Schlotter, The lead dioxide reserve type cell. J. Electrochem, Soc. Vol. 99, No. 8) or batteries of the type of potassium bisulfate-lead dioxide (J. C. Duddy, U.S. Pat. No. 3,466,194 of Sept. 9, 1960) in which the electrolyte is only contacted with the plates at the moment in which the cell is to be used, in view of the aggressivity of the acid medium over the zinc, even at open circuits. On the other hand, these cells are irreversible, that is, are not rechargeable, because under these conditions, the charging process causes many technical problems which have not been overcome, such as high hydrogen evolution which prevents electroplating of the zinc on the anode, "treeing" of the zinc deposits which causes short circuiting between the electrodes, and the necessity of a high degree of amalgamation of the zinc plates to prevent formation of localized cells, which renders said plates brittle, therefore rendering the system useless as an efficient electrical energy generating system.
Schlotter, therefore, very clearly indicates that because of the reactiveness of zinc in sulfuric acid, which is the common electrolyte previously used in this type of cells, accompanied by the evolution of hydrogen gas, it is necessary to store the lead dioxide-zinc type cells in a dry condition and to introduce the electrolyte when the cell is put into use. While this may be accomplished by means of a self contained chamber electrically or mechanically operated, or by introducing the electrolyte from an external source, the problems involved in these operations are rather intricate and, therefore, have contributed to discourage use of this type of lead dioxide-zinc reserve cells.
In view of the above, all the lead dioxide-zinc reserve type cells of the prior art have been considered to be highly disadvantageous in view of the fact that the operation of the above mentioned couple causes high gas evolution and poor low temperature characteristic, which render this type of cells rather useless in practice as an electrical generating unit.